1. Field of the Invention
This invention relates generally to refrigeration and heat exchange. More specifically, it relates to a refrigeration and heat exchange system and process for cooling a fluid product in a stream process, and to improved heat exchange in such a system.
2. Background Information
Numerous products of a fluid nature, including a wide variety of food products, are processed and produced in stream processes at temperatures higher than the desired temperature at which the product is to be packed, bottled, canned, or the like. The product must therefore be cooled to the desired temperature in order to insure its proper packing. Many fluid food products, including relatively viscous liquids (e.g. mustard, ketchup, syrup) and free flowing liquids (e.g. beer, milk, vinegar) are processed in this way.
In further describing this background, and the invention itself, it will be helpful to use a specific application of the invention to illustrate the discussion. Dairy processing of milk is a good example for this purpose.
A typical dairy system includes a raw milk storage tank, pasteurizer, homogenizer, cooler, and container filler, all connected in series for continuous stream processing. After leaving the storage tank, the milk is pasteurized at an elevated temperature, cooled in the cooler to 40.degree. F. or below, then dispensed into containers of various sizes at a temperature close to the cooler-exit temperature. In typical current systems the temperature of milk in containers immediately after filling is approximately 42.degree. F. to 44.degree. F. in half gallon containers, and approximately 52.degree. F. to 54.degree. F. in half pint containers. Filling and closure of the containers requires application of heat. The combination of this added heat and the different milk quantities in the different size containers accounts for the substantial differences in milk temperatures in the larger and smaller containers.
It is desirable to fill, store, and deliver milk at as low a temperature, without freezing, as possible. It is an object of this invention to attain product temperatures substantially lower than has been attainable in the prior art. In the case of milk processing, the present invention reduces final product temperatures from the figures given above to approximately 34.degree. F. in half gallon containers and approximately 39.degree. F. in half pint containers.
U.S. Pat. Nos. 1,853,724 to Davenport, and 3,636,723 to Kramer are somewhat relevant prior art. Davenport discloses a refrigeration system including compressor, condenser, injector nozzle, evaporator, and separator, all connected in series. The heat transfer medium in Davenport is an emulsion of refrigerant and air or other non-condensible gas, this for a particular purpose not relevant here. Kramer discloses a refrigeration system which includes a defrosting line 36, by-passing the condenser 31 and expansion valve 33, to inject heat directly into the evaporator 30 for defrosting the evaporator.
Another aspect of this invention relates to heat exchange. In a heat exchanger, a first fluid medium gives up heat to a second fluid medium through a thermally conductive barrier which separates the two media. If the heat exchange media are liquids, and if they move relative to the barrier in a laminar flow condition, the boundary layer of each liquid near the barrier surface is at or near zero velocity and the temperature of that layer is at or near the temperature of the barrier. Successive layers or "lamina" are at respective incremental temperature differences relative to the barrier temperature. These thermal gradients on each side of the barrier offer considerable resistance to heat transfer from one medium to the other. The lamina and their resulting thermal gradients and resistance to heat transfer become especially significant if one or both of the heat exchange media are viscous fluids. Generally, the way to remove or reduce this resistance is to continually remove and replace the boundary layers with fluid from the "main stream".
In process systems of the prior art, the fluid product is typically conveyed through a wiped film heat exchanger. The boundary layer of product fluid is removed, and heat exchange aided, by continual wiping of the product fluid from the heat exchange surface by a wiping blade. Such mechanical wiping apparatus is in intimate contact with the product fluid and must be cleaned periodically, usually by a more or less complex and costly disassembly of the apparatus and replacement of parts.